When delivered orally, most non-invasive and non-replicating antigens are not only poorly immunogenic but likely to induce an antigen-specific system hyporesponsiveness termed oral tolerance. The mechanism of oral tolerance is not fully understood but appears to be due to a deficit in systemic T cell help. Thus, despite the fact that oral vaccines are desirable for their ease of administration, few exist.
In rabies vaccination, however, both live-attenuated rabies and vaccinia-rabies glycoprotein recombinant virus delivered in edible bait have proven to be effective vaccines. Since rabies virus infection initiates from skin or muscle by a bite, it is clear that the ingested bait-vaccines induce a systemic immune response rather than a mucosal response and systemic tolerance which often accompanies the feeding of antigen. It has been proposed that the ability of the live vaccines to replicate is important in generating systemic immunity when administered orally. However, there is recent evidence to suggest that ingested killed rabies virus or rabies virus proteins also induce protective immunity.
Although virus neutralizing antibodies are invariably directed at the rabies glycoprotein, several previous studies have indicated that protective immunity to infection with rabies virus can be engendered by the intra-muscular administration of the rabies virus ribonucleocapsid. Dietzschold et al. (WO 8900861 A) disclose a method of inducing immunity to a rabies-related virus comprising administering a priming injection and one or more booster injections containing an antigen and a polypeptide having sequence homology with a rabies-related virus nucleoprotein. Antigenic determinants borne by the N protein of the rabies virus ribonucleocapsid have been proposed to play a role in the immunoprotective effect of this rabies vaccine.
Immunological enhancers, such as adjuvants, are substances which are added to vaccines or to antigens used for immunization to stimulate the immune response. Adjuvants cause an accumulation of mononuclear cells, especially macrophages, at the site of injection that is a granulomatous response. Macrophages involved in this first stage of the immune response take in the protein antigens and break them down into peptide fragments which are then exposed on the cell surface where they form a physical association with class II histocompatibility antigens. The T helper cells only recognize proteins associated with class II histocompatibility antigen, and not the free undegraded protein. The macrophages release monokines from the interleukin-1 family (IL-1) which stimulate the T helper cell to secrete IL-2. The actions of IL-1 and IL-2 result in the clonal expansion of T helper cells. The clonal expansion of T helper cells is followed by their interaction with B-cells, which in turn secrete antibody.
It has now been found that the nucleocapsid and ribonucleoprotein complex of a virus enhances the immune response in vivo to an antigen when administered in combination with the antigen. This enhanced immune response has been observed following various routes of administration including oral. The immunological enhancement by ribonucleocapsid complexes make them especially useful in development of effective oral vaccines.